Multichannel Seismic Lines And Odp Results

On regional multichannel seismic (MCS) lines collected in 1985 and 1989, a hydrate BSR was observed in a 20-30 km wide band along much of the 250 km length of the Vancouver Island continental slope (Figs. 1, 2, 3). ). A BSR is generally not observed in the well-bedded slope basin sediments (Fig. 3a), but interpretations vary as to whether or not hydrate is present. Hydrate may not be formed because the well-bedded sediments reduce permeability, thus inhibiting vertical fluid and methane flow (Zuehlsdorff et al., 2000); alternatively, hydrate may be present but tectonic subsidence in the basin results in downward movement of the base of the stability field, so that the gas layer is transformed to hydrate and the BSR is much weakened (von Huene and Pecher,

The BSR signal is generally a single symmetrical wavelet with a reversed polarity relative to the seafloor, indicating a sharp and negative impedance contrast across the BSR (Figs. 3, 4). The seafloor reflection coefficients are typically 0.18-0.24, and the BSR reflection coefficients are about 30% those of the seafloor (Yuan et al., 1996). Careful semblance velocity analyses were carried out on sporadic reflectors down to depths of 2000 mbsf (Fig. 5a). Above the BSR, velocities increase to nearly 1900 m/s, indicating the presence of sediments containing high-velocity hydrate within the pores. However, the critical importance of these offset [km]

offset [km]

Figure 4. Common-depth-point gather after normal moveout and array directivity corrections from Line 89-08. The gather illustrates the seafloor and BSR amplitude-versus-offset (AVO) behavior.

measurements is that they provide the only estimate available for velocities below the BSR and gas layer. By extrapolating these deeper velocities upwards, we thus obtain the reference velocity of sediments unaffected by either hydrate or free gas. At the BSR, hydrate produces an increase in velocity of about 250 m/s.

Downhole sonic logs from ODP Site 889 (Westbrook et al., 1994) provide detailed velocity information from about 50 metres below the seafloor (mbsf) to the BSR at 225 mbsf (Fig. 5b). Excellent agreement with the semblance velocity results was obtained. The best constraint for low velocities below the BSR, due to the presence of small quantities of gas, comes from a vertical seismic profile (VSP) at Site 889 (MacKay et al., 1994); unfortunately, these measurements are limited to a depth of only about 30 m below the BSR, and so no conclusions can be made about the thickness of the gas layer.

Semblance Velocity Profile

Figure 5. (a) The MCS velocities along L89-10 define the velocity trend down to depths of nearly 3000 mbsf. (b) Sonic logging and vertical seismic profile for Site 889 are shown in comparison with MCS velocities. The solid line fitting the velocities represents the reference velocity profile extrapolated from the deep trend.

Additional information on velocity structure is contained in the phase and amplitude variations with offset for a given common midpoint gather (Fig. 4). The observed amplitude behavior for the BSR, when corrected for the pronounced directivity of the airgun array, shows constant or slightly decreasing amplitudes at near-to-mid offsets and a large amplitude increase at far offsets. Modeling of the amplitude variations indicates that a P wave velocity increase is

Figure 5. (a) The MCS velocities along L89-10 define the velocity trend down to depths of nearly 3000 mbsf. (b) Sonic logging and vertical seismic profile for Site 889 are shown in comparison with MCS velocities. The solid line fitting the velocities represents the reference velocity profile extrapolated from the deep trend.

Additional information on velocity structure is contained in the phase and amplitude variations with offset for a given common midpoint gather (Fig. 4). The observed amplitude behavior for the BSR, when corrected for the pronounced directivity of the airgun array, shows constant or slightly decreasing amplitudes at near-to-mid offsets and a large amplitude increase at far offsets. Modeling of the amplitude variations indicates that a P wave velocity increase is the main cause of the BSR amplitude increase at large offsets (Yuan et al., 1999). The S wave velocity in hydrate was assumed to increase in proportion to the P wave velocity increase, as may be expected if hydrate cements sediment grains; this is consistent with the increase observed on S wave dipole logs in drilling in the Beaufort Sea (Lee and Collett, 1999) and on the Blake Ridge (Guerin et al., 1999).

Subtle differences in reflection waveforms provide additional constraints on the detailed velocity profile through careful full waveform inversion (Singh and Minshull, 1994; Yuan et al., 1999). The inversion is very dependent on the starting model, which was derived from the sonic, VSP, and semblance velocity profiles. The results indicated that velocities as low as 1500 m/s occur in a 25-50 m thick layer below the BSR (Fig. 5b). Based on the reference velocity estimate at the BSR of 1650 m/s, we concluded that the BSR in this area is consistent with a model described by a velocity increase due to hydrate above the BSR and a velocity decrease due to free gas below the BSR.

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